/Engine/Source/Runtime/GeometryCore/Public/Implicit/SweepingMeshSDF.h
The narrow band is always computed exactly, and the full Grid is always signed. Can also fill in the rest of the full Grid with fast sweeping. This is quite computationally intensive, though, and not parallelizable (time only depends on Grid resolution)
enum EComputeModes { FullGrid = 0, NarrowBandOnly = 1, NarrowBand_SpatialFloodFill = 2, }